The present invention pertains to digital transmission systems and more particularly to maintaining proper framing between multiple copies of time multiplexed digital span equipment.
Modern telecommunication systems transmit large amounts of data rapidly between systems. Digital spans are utilized to connect these systems and to transmit this data. The switching system usually contains a number of digital spans, each span transmitting data at a high rate between switching systems.
For public policy reasons, these switching systems must be highly reliable. In order to achieve this reliability, these systems are often implemented with redundant equipment. This redundancy scheme is a way of providing for fault tolerance of the system. That is, if one portion of the system fails there is a second portion of the system performing exactly the same process. The system functions properly, but in a simplex mode.
The redundant equipment is typically operated in a synchronous fashion. This prevents total system down time as a result of switch-over from one copy of the equipment to the other. It also provides a means of fault detection if the two copies differ.
U.S. Pat. Nos. 4,531,210 and 4,507,780 deal with duplex digital span controllers for one such system. This system is the GTD-5 EAX manufactured by GTE Communication Systems Corporation, the assignee of the present application. These patents teach duplex digital control units which operate synchronously. However, these digital control units serve a number of digital spans and therefore require a multiplexing operation. Because of the high-speed data and the nature of scanning, signal propagation times through similar circuits may vary. That is, at a particular instant, one copy of a circuit may see a framing bit set and the other duplex copy of that circuit may not see the corresponding framing bit set. Therefore, synchronization is difficult to maintain in multiplexed reframing operations. Since finding proper framing is a trial and error process, quickly locating framing and synchronization bits will aid in maintaining synchronization of the duplex digital span equipment.
Accordingly, it is an object of the present invention to provide an arrangement for maintaining proper framing between multiple copies of time multiplexed digital span equipment.